1. Field of the Invention
The present invention relates generally to an optical transceiver module, and in particular to an optical subassembly of the optical transceiver module, featuring small size, high precision alignment, and excellent coupling efficiency.
2. The Prior Arts
Optical fiber transmission is instrumental in the development of many advanced applications for telecommunications and data communications. This high bandwidth transmission needs local fiber access to provide two-way communications to the home through an optical transceiver, which is composed of a driver circuit, an electrical subassembly (ESA), and an optical subassembly (OSA). Operation frequency plays a critical role in determining the transmission speed. Conventionally, high speed transmissions up to 10 Gb/sec, 40 Gb/sec, or even higher, is realized by increasing the operation frequency of the transmitter driver circuit, which inevitably leads to a significant increase of manufacturing costs.
Wavelength Division Multiplexing (WDM) was developed to enhance the transmission speed of optical fiber system without undesirably increasing the operation frequency and thus effectively limiting the increase of manufacturing costs. The WDM solution allows an optical transceiver to multiplex a plurality of optical signals of different wavelengths onto a “mixed” signal that can travel along a single optical fiber. Such a “mixed” signal, once reaching a destination receiver, is demultiplexed and separated with the constituent component of the desired wavelength retrieved. In other words, the WDM technology optimizes the utilization of transmission bandwidth by permitting simultaneous transmission of optical signals of different wavelengths over a single optical fiber. Two types of WDM are known, namely Dense Wavelength-Division Multiplexing (DWDM) and Coarse Wavelength-Division Multiplexing (CWDM), based on the minimum size of the spacing between wavelengths of the optical signals that can be composed into the single “mixed” signal.
For DWDM, the normal spacing between two bands of different wavelengths is in the range of 0.8–1.6 nm, so that unitary bandwidth can support extremely high optical signal density. C-band that operates in a bandwidth of 1525–1565 nm is most commonly used for long haul, MAN and LAN signal transmission. Due to the dense arrangement of optical signals in a single band, optical splitters and photo coupler modules that are employed for DWDM must be upgraded. In addition, a thermo-electric cooler (TEC) that is expensive is needed to control the operation temperature of a laser diode that emits the desired optical signals whereby micro-drifting of the wavelength of the optical signals can be eliminated to ensure transmission quality. All these add to the manufacturing costs, as well as power consumption.
On the other hand, Coarse Wavelength-Division Multiplexing (CWDM) arranges less optical signals in a single optical fiber, which allows for a large spacing (20 nm) between wavelengths of the optical signals. This wavelength spacing is much larger than that of the DWDM. Thus, CWDM does not require the expensive thermoelectric cooler (TEC) to reduce the operation temperature of the laser diode nor to prevent the drifting of the bandwidth.
Although CWDM has a transmission capacity lower than DWDM, such a drawback can be easily overcome by using a number of laser diodes of lower transmission speeds employing CWDM to simultaneously transmit optical signals, and a high-speed transmission device compared with DWDM can be realized. For example, to meet a transmission requirement of 10 Gb/sec, CWDM only needs several laser diodes with lower transmission speed, for example laser diodes of 3.125 Gb/sec or 2.5 Gb/sec, which by the nature thereof are more stable in signal transmission, to produce the equivalent performance as a laser diode of 10 Gb/sec. As another example, to reach up to 40 GB/sec transmission, four laser diodes of 10 Gb/sec or slightly higher baud rates together can meet the required specifications. This method can be expanded for even higher transmission bandwidths.
The use of laser diodes of the transmitter optical subassembly (TOSA) with lower transmission speeds allows the sensing area of the corresponding photo diodes on the receiver optical subassembly (ROSA) to be increased. Therefore, the alignment tolerance is less critical and the coupling efficiency between optical signals and optical fiber can be improved.
Also, using laser diodes with lower transmission speeds makes the design for the electrical subassembly (ESA) and driver circuit less critical, but the more challenging part is the design of the optical subassembly (OSA), which is to combine optical signals of different wavelengths and couple them onto a single optical fiber (the part of TOSA), or to separate multiplexed optical signals on the receiver end of the optical fiber into optical wavelength signals to respective photo detectors (the part of ROSA), and at the same time the design spec has to meet the Multi Source Agreement (MSA) and the module miniaturization.
Coupling efficiency for a number of optical signals of different wavelengths is of vital importance in reducing signal loss in optical transmission and in reducing the misalignment among each component. Passive alignment is commonly employed to simplify the manufacturing process. The passive alignment is done by forming a mating portion on the body of an optical subassembly. The mating portion is machined with high precision. An optical device with a counterpart mating portion, which is also precisely machined, is inter-engaging with the mating portion of the body. Since both mating portions are of high machining precision, the coupling efficiency is enhanced. However, since machining precision is subject to limitation, the improvement of coupling efficiency is also subject to limitation. Thus, for precise alignment, the mating means must be of extremely high manufacturing precision and this inevitably complicates the manufacturing process and increases the manufacturing costs.
On the other hand, an active alignment technique allows for adjustment of the position of an optical device with respect to an optical transmitter or receiver in order to obtain an optimum coupling therebetween after the optical device is mounted to the transmitter or receiver. An example of active alignment is illustrated in U.S. patent application Ser. No. 10/971,462 and its Taiwanese counterpart, Taiwan Patent Application No. 93118803. The coupling efficiency can be optimized by means of the after-mounting adjustment and flexibility can be provided for manufacturing/assembling of the optical transmitter and receiver.
For an optical transmitter or receiver that transmits or receives a number of optical signals of different wavelengths, a number of laser diodes or photo detectors. Each laser diode or photo detector must have an individual base for independent adjustment, which leads to a bulky size of the optical transmitter or receiver.
Thus, the present invention is aimed to provide an optical subassembly that overcomes the above-discussed drawbacks of the conventional optical subassemblies.